Autoimmune diseases result from the recognition of "self" by the immune system followed by a humoral (antibody) or cell-mediated response that leads to the destruction of the one's own cells. While a healthy immune system selects against self-reactive immune cells (T cells and B cells) in the thymus by quickly destroying them, this system is imperfect. When self-reactive cells are released into the circulation and penetrate into peripheral tissues they may encounter the self antigen to which they can respond. These antigens are displayed on the surface of cells in the form of peptide fragments, non-covalently associated with either class I or class II antigen molecules of the major histocompatibility complex (HLA in humans). Fortunately, this first encounter of self reactivity generally results in a weak response. This is because multiple signals are required to stimulate a proliferative response that both activates the effector function of the cell and increases its number by cell division. Without a second co-stimulatory signal, which is found on professional antigen-presenting cells (APC), the T cell becomes non-reactive (or anergic) to a second exposure to the same antigen and a harmful reaction is prevented.
How this protective system breaks down in autoimmune disease is under intense investigation and many associations have been made between specific types of immune responses and disease activity. For example, conditions that lead to the up-regulation of HLA molecules in the central nervous system (CNS), and increased antigen presentation, have been associated with the T-cell mediated destruction of nervous tissue in multiple sclerosis (MS) patients. While the exact triggering event is unknown, a clear picture is emerging as to how the immune system regulates such responses. One of the key immune regulators is the T helper cell which reacts to antigens presented on HLA class II molecules. This CD4.sup.+ cell differentiates in response to antigenic stimulation and becomes a type 1 or type 2 helper (Th1 or Th2) according to the type of cytokines that it secretes (Mosmann and Coffman, Ann. Rev Immunol. 7:145). A Th1 response leads to the secretion of interleukin-2 (IL-2) and interferon-.gamma. (IFN-.gamma.) which stimulates cell-mediated immune reactions against intracellular pathogens. A Th2 response leads to the secretion of IL4, IL-5 and IL-10 which stimulates antibody responses to extracellular pathogens. The most interesting component of this system of regulation is that one response inhibits the other through the negative regulatory activities of the cytokines that are produced. Thus, IL-4 and IL-10 can down-regulate Th1 responses while IFN-.gamma. can down-regulate Th2 responses.
The importance of such a regulatory feedback loop in autoimmune disease recently has been associated with disease activity in multiple sclerosis. T cells cloned from patients undergoing active disease have been shown to produce TH1 cytokines upon stimulation with antigen in vitro (Correale et al. J. Immunol. 154:2959). T cell clones obtained from the same patient during the remission phase produce Th2 cytokines. They also produce another potent suppressor of cell-mediated immunity, tumor growth factor .beta. (TGF-.beta.).
The regulatory activity of T helper cells and their differentiation following exposure to antigen is regulated by cytokines as well. IL-12 has been shown to be essential in the generation of Th1 cells. IL-12 is released primarily by the antigen presenting cell, which for Th1 responses is normally a macrophage (reviewed by Trinchieri, Blood 84:4008). Other cytokines also are secreted by the responding T cell after antigen stimulation, especially IL-2. Cytokines IL-12 and IL-2 have a powerful synergistic effect in the induction of IFN- from both T helpers and natural killer (NK) cells (Eur. Patent Appl. 90123670.3). This secreted IFN-.gamma. then inhibits any Th2 cell proliferation and polarizes the response to favor cell-mediated immunity (the Th1 response). If IL4 was the major cytokine present during antigen stimulation, a Th2 response would be made and the Th1 response would be inhibited. Thus, the initiating event that establishes the cytokine environment has an important role in determining the nature of the immune response. Another consequence of the Th phenotype is reflected in the isotype of antibody that is made in response to an antigen. TH1 responses lead to increases in cytolytic antibodies, i.e., those capable of mediating antibody-dependent cellular cytotoxicity (ADCC) and those that activate the complement system. Th2 responses lead to the production in non-cytolytic classes (isotypes) of antibodies. The importance of this phenomenon has recently been described in mouse models of collagen-induced arthritis where Th1 responses induced by IL-12 favored the production of IgG2a (cytolytic) over IgG1 (non-cytolytic) and this class switching correlated with disease occurrence and severity (Germann et al., Proc. Natl. Acad. Sci. 92:4823).
It is one object of this invention to provide compositions for antagonizing the IL-12 induced activities of immune cells. It is another object of this invention to provide a method for antagonizing IL-12 induced activities of T helper cells so as to inhibit the IFN-.gamma. induced Th1 response, be effective to modulate the induction of Th2 cells; be effective to inhibit a cellular immune response; and/or be effective to stimulate the production of Th2 cytokines including IL-4, IL-5, and L-10. It is still another object of the invention to provide a method for stimulating the cellular production of cytokines in immune cells, which can be immune cells in a mixed or selected population of cells in an in vitro cell culture or can be circulating immune cells in a mammal. It is still another object of the invention to provide an in vitro diagnostic for measuring IL4 production in peripheral blood mononuclear cells.